1. Field of the Invention
The present invention relates to an OFDM receiving method and an OFDM receiver, for realizing multiplexed communications between a transmitter and a receiver via a plurality of paths obtained by space division.
2. Description of the Background Art
In recent years, an OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme, being a type of a multi-carrier transmission scheme, has been employed for wireless LAN, and the like, as a modulation scheme with a high resistance to frequency selective fading, which occurs under a multi-path environment in mobile communications. In an attempt to further improve the frequency efficiency, methods have been proposed in the art, which use a plurality of transmitter antennas and a plurality of receiver antennas to form MIMO (Multi Input Multi Output) channels, thereby realizing multiplexed communications between a transmitter and a receiver via a plurality of paths obtained by space division. The receiver side estimates the inverse functions for the propagation coefficients of a plurality of paths based on signals from the receiver antennas and equalizes the received signals, thereby separating the transmit signals from different transmitter antennas. Thus, it is possible to realize as many channels as there are transmitter antennas.
An MIMO-OFDM modulation scheme has been proposed in the prior art, e.g., Japanese Patent 3590008, which is a combination of OFDM, which is resistant to multi-path environments, and MIMO, which can improve the frequency efficiency. FIGS. 18 and 19 show configurations of an OFDM transmitter 200 and an OFDM receiver 220, respectively, which use a conventional MIMO technique as disclosed in this patent document. FIGS. 18 and 19 show a 2×2 MIMO-OFDM configuration where there are two transmitter antennas and two receiver antennas.
Data modulated through a data modulating section 201 is divided into a portion for a transmitter antenna 206 and another portion for a transmitter antenna 207, and these portions are OFDM-modulated through OFDM modulating sections 202 and 203, respectively. In this process, signals necessary in the signal receiving process, such as preambles 601 and 602 (needed for synchronization) and training symbols 603 and 604 (needed for estimating propagation coefficients), are added, thereby forming a transmit frame 1 and transmit frame 2 (FIG. 20). The transmit frame 1 and the transmit frame 2 are converted to radio frequencies through frequency converting sections 204 and 205, and are transmitted from the transmitter antennas 206 and 207.
Signals transmitted from the plurality of transmitter antennas 206 and 207 arrive at a plurality of receiver antennas 208 and 209 via different paths. The propagation coefficient between a transmitter antenna and a receiver antenna is herein denoted as hj,i, where i is the transmitter antenna number and j is the receiver antenna number. In the case of 2×2 MIMO, there are four transmission paths: h1,1, h1,2, h2,1 and h2,2. Then, the relationship between a transmit signal Si and a receive signal Rj is represented by Expressions (1) and (2) below.R1=h1,1×S1+h1,2×S2  (1)R2=h2,1×S1+h2,2×S2  (2)
If the propagation coefficients hj,i are uncorrelated to each other and the inverse function of hj,i can be obtained, it is possible to separate the transmit signals from the multiplexed receive signal. This can be achieved by, for example, obtaining the inverse matrix of a propagation matrix H whose elements are hj,i and then multiplying a matrix R of the receive signal Rj by the inverse matrix. Specifically, where S=[S1, . . . , SN]T denotes a transmit signal matrix whose elements are the signals Si transmitted from a number N of transmitter antennas, R=[R1, . . . , RM]T denotes a receive signal matrix whose elements are the signals Rj received by a number M of receiver antennas, and H=hj,i denotes a propagation matrix whose elements are a number M×N of propagation coefficients hj,i between the transmitter and receiver antennas, a receive signal R is represented as shown in Expression (3) below.R=HS  (3)
Multiplying each side of Expression (3) by W═H−1, which denotes the inverse matrix of the propagation matrix H, yields WR=WHS=H−1HS═S. Thus, it is possible to separate transmit signals S from each other.
On the receiver side, radio signals received by the receiver antennas 208 and 209 are converted by frequency converting sections 210 and 211, respectively, to frequency bands suitable for signal processing operations. The converted receive signals are OFDM-demodulated through OFDM demodulating sections 212 and 213, and separated into a plurality of subcarrier signals as shown in FIG. 21. A transmission path estimating section 214 estimates the propagation coefficient hj,i for each path by using the training symbol added for the purpose of estimating the propagation coefficient. An inverse matrix calculating section 215 obtains the inverse matrix of the propagation matrix H whose elements are hj,i. An interference canceling section 216 performs an interference cancellation operation for the receive subcarrier signals by using the inverse matrix of the propagation matrix H, thereby separating the multiplexed transmit signals from each other (channel separation) The separated transmit signals are demodulated through a data demodulating section 217.
Using the synchronization preambles, the OFDM demodulating sections 212 and 213 perform carrier wave frequency synchronization, clock synchronization and symbol synchronization, and corrects frequency errors and timing errors. Then, the time axis signal is converted to a frequency axis signal and is divided into subcarrier signals.
If a synchronization estimation error occurs, there will be a phase error in the subcarrier signals. If the phase error increases, it causes a demodulation error. In view of this, in a conventional OFDM transmission operation, a particular subcarrier is transmitted while being assigned a known phase (pilot carrier) so that the phase error can be estimated/corrected by using the received pilot carrier.
FIG. 22 shows an example of an MIMO-based OFDM receiver 230 with conventional phase error correction. Like elements to those shown in FIG. 19 will be denoted by like reference numerals and will not be further described below. A pilot extracting section 501 extracts the pilot carrier from among the subcarriers, which are obtained by channel separation through the interference canceling section 216. A phase error estimating section 502 estimates the phase error from a comparison between the phase of the extracted pilot carrier and the known phase used at the time of transmission. A correction section 503 corrects the data carrier so as to correct the estimated phase error, and the data demodulating section 217 demodulates the data.
The precision of the interference cancellation can be improved by repeating such a demodulation operation a number of times, as follows. The data demodulated through the data demodulating section 217 is re-modulated through a data modulating section 504 to produce a transmit signal. A replica producing section 505 multiplies the re-modulated transmit signal by the estimated propagation coefficient to produce a replica signal. The replica signal, regarded as an interfering signal, is subtracted from the receive signals, and the remaining signals are successively separated. There may be provided as many such demodulation stages 506 and 507 as needed.
However, simply combining OFDM modulation scheme with MIMO-based transmission path estimation, as in the MIMO-based OFDM receiver 220 disclosed in the above-specified patent document, poses problems as follows. Multiplication with the inverse matrix of the propagation coefficient matrix in the channel separation operation normalizes the amplitudes of the separated signals, irrespective of the original reception level. As a result, the noise level of a signal originally having a low reception level will be emphasized by the amplitude normalization. Therefore, if the phase error is obtained from the separated pilot carriers, the error in the estimation result may become significant due to the noise emphasis. Thus, it is not possible to properly correct the phase of the separated data carriers, resulting in a demodulation error.
With a configuration, such as that of the MIMO-based OFDM receiver 230 with phase error correction, which performs repeated demodulation by producing a replica signal and subtracting the replica signal from the receive signals, it is necessary, for each demodulation stage, to extract the pilot carrier and correct the phase error. Therefore, the receiver will become complicated and large in size.